What happens to the IR radiation that the Greenhouse gases don't absorb?

[Jyrioffinland]

TL;DR

The lower atmosphere absorbs green house gasses at certain frequencies, thus the Earth emits black body radiation at the temperature of -9F (-23C). But there are frequencies that CAN jump straight to space. Why don't they raise the BB temperature?

I have been searching for the answer yet – as I'm here – didn't find it.

The current model of climate warming says the lower parts of Earth's atmosphere don't allow the IR radiation to escape freely to the outer space because the GH gas molecules keep absorbing it virtually as soon as it's been emitted by another GH gas molecule.

That means the Earth can give out the excess heat by BB radiation only at higher altitudes (5+km), where there is no longer enough GH gasses present to block it. Also, the temperature there is much lower, thus there the BB radiation is less energetic, corresponding to the temperature of -9°F (-23°C).

But what happens to the near-surface IR radiation at the frequencies that can NOT be absorbed/emitted by the GH gasses? Shouldn't it raise the Earth's actual BB temperature?

Is it because it makes no (substantial) difference and can be excluded from the equations? Or is it just been omitted from the public (non-scientific) discussions to make it easier to understand?

[Note: Pop-sci YouTube link deleted by the Mentors]

 

[Baluncore]

But what happens to the near-surface IR radiation at the frequencies that can NOT be absorbed/emitted by the GH gasses? Shouldn't it raise the Earth's actual BB temperature?

It does raise the BB temperature, to -23° C.

 

[Jyrioffinland]

So the non-scientific articles ignore this in order to keep it simple, I reckon.

 

[DrClaude]

I'm not sure I understand the question. The Earth is not a perfect blackbody. The emitted spectrum only follows approximately that of a black body, so what role does it play if there are peaks at certain IR frequencies due to the transparency of the atmosphere?

Edited to add this figure from F. W. Taylor, Elementary Climate Physics (OUP, 2005)

 

[Baluncore]

But what happens to the near-surface IR radiation at the frequencies that can NOT be absorbed/emitted by the GH gasses?

Hot material radiates across the IR spectrum with a Planck spectrum distribution.
https://en.wikipedia.org/wiki/Black-body_radiation#Spectrum
Some of that will be absorbed by GH gasses, while other wavelengths have a chance of escape into space. The air and GH gasses, may be transparent to those wavelengths, but aerosols such as clouds, pollutants, dust and pollen are not, so will be heated.
https://en.wikipedia.org/wiki/Aerosol#In_the_atmosphere

Once those aerosol materials are heated, they will lose heat by conduction to the air, or radiate the thermal energy, again with a Planck distribution. For that reason, wavelengths that might escape, can be converted by aerosols, to wavelengths that will be absorbed by GH gasses.

 

[Bruzote]

From what I am reading, I think you are confusing emission and absorption. The reality is that airborne and spaceborne sensors are simply measuring all of the emitted radiation that reaches them. They don't measure atmospheric absorption. That can be inferred later. Some of that radiation arrived directly from the ground/ocean/ice (passing through the atmosphere), while some of it is emitted by the atmospheric constituents, including gases and aerosols (with precipitation in there as well).

Of course, the purpose of measuring radiation is generally to determine the ACTUAL temperature of all parts of the Earth system, whereas BB "temperature" is an abstract construct that *might* be used as an intermediate variable to calculate temperature. So, why wouldn't they ignore that in what they report? They are not going to show intermediate calculations in most of their work.

Now, if you're curious about how the scientists convert the radiation measurements to estimate of actual temperature, they use formulas/models which DO include parameters about the emissivity/absorptivity of the whole atmosphere and the surface beneath it. The parameters are dependent on radiation frequency and the atmospheric pressure (due to "pressure broadening"). So, the calculations are run for various heights in the atmosphere and for many narrow frequency bands covering the range of infrared radiation. The models have very detailed frequency-dependent parameters for *many* gases in the atmosphere, including trace gases you might not think about (e.g., O3, SO2, NO2, etc.). The detailed models also break down the atmospheric emissions by altitude layers because mixing ratios of each gas can vary by altitude, with production and destruction rates for photochemically active molecules varying with changes in light and UV radiation as well as pressure and other co-reactive species. Also, in the stratosphere and above, there tends to be less turbulence so lighter gases tend to rise and heavier gases tend to sink, which can actually affect the mixing ratios of molecular species for each model layer. Scientists are not leaving out the things you worry about. Let them worry about it. They've been improving on this stuff for a CENTURY. You are not about to have a "a-ha" moment and find something they've omitted.

Once the scientists have verified and tested a model that consistently converts a certain clear-sky flux measurement at a certain sun angle to a certain surface temperature, they do need not run the model each time. Once they have consistent results, they can use it to directly map a measured flux at the sensor to a particular surface temperature. That is how they can quickly turn around each measurement into a map of satellite-measured surface temperature. (They actually do this for the whole depth of the atmosphere, called "profiling".)

 

[Jyrioffinland]

From what I am reading, I think you are confusing emission and absorption. The reality is that airborne and spaceborne sensors are simply measuring all of the emitted radiation that reaches them. They don't measure atmospheric absorption. That can be inferred later. Some of that radiation arrived directly from the ground/ocean/ice (passing through the atmosphere), while some of it is emitted by the atmospheric constituents, including gases and aerosols (with precipitation in there as well).

Of course, the purpose of measuring radiation is generally to determine the ACTUAL temperature of all parts of the Earth system, whereas BB "temperature" is an abstract construct that *might* be used as an intermediate variable to calculate temperature. So, why wouldn't they ignore that in what they report? They are not going to show intermediate calculations in most of their work.

Now, if you're curious about how the scientists convert the radiation measurements to estimate of actual temperature, they use formulas/models which DO include parameters about the emissivity/absorptivity of the whole atmosphere and the surface beneath it. The parameters are dependent on radiation frequency and the atmospheric pressure (due to "pressure broadening"). So, the calculations are run for various heights in the atmosphere and for many narrow frequency bands covering the range of infrared radiation. The models have very detailed frequency-dependent parameters for *many* gases in the atmosphere, including trace gases you might not think about (e.g., O3, SO2, NO2, etc.). The detailed models also break down the atmospheric emissions by altitude layers because mixing ratios of each gas can vary by altitude, with production and destruction rates for photochemically active molecules varying with changes in light and UV radiation as well as pressure and other co-reactive species. Also, in the stratosphere and above, there tends to be less turbulence so lighter gases tend to rise and heavier gases tend to sink, which can actually affect the mixing ratios of molecular species for each model layer. Scientists are not leaving out the things you worry about. Let them worry about it. They've been improving on this stuff for a CENTURY. You are not about to have a "a-ha" moment and find something they've omitted.

Once the scientists have verified and tested a model that consistently converts a certain clear-sky flux measurement at a certain sun angle to a certain surface temperature, they do need not run the model each time. Once they have consistent results, they can use it to directly map a measured flux at the sensor to a particular surface temperature. That is how they can quickly turn around each measurement into a map of satellite-measured surface temperature. (They actually do this for the whole depth of the atmosphere, called "profiling".)

Wow! Thank you a lot!

And I was NOT looking for an "a-ha" moment but better understanding of the phenomena.

 

[Bruzote]

Wow! Thank you a lot!

And I was NOT looking for an "a-ha" moment but better understanding of the phenomena.

Sorry. My assumption was improper. :-b

 

[Jyrioffinland]

Sorry. My assumption was improper. :-b

My bad. I should have used the term "popular" in stead of "public".